Continuously variable cone pulley transmissions for motor vehicles are known in the prior art. An example is the transmission shown in German Pat. No. 30 28 490, in which a part of the primary pulley assembly is axially displaceable by the fluid displacement piston of a hydraulic displacement servo. The displacement piston of the displacement servo is connected in an axially rigid manner to the driving shaft of a continuously variable cone pulley transmission. It is surrounded by a displacement cylinder secured to the axially displaceable pulley part.
To produce the desired clamping forces as a function of the load, the primary shaft that supports the primary pulley is provided with a sensor for the torque transmitted by it. This torque sensor comprises a driving collar that is axially rigid with respect to the primary shaft but rotatable relative thereto. It is connected by way of meshing teeth to a rotary ring, the front side of which forms an annular piston received in an annular cylinder in the rigid cone pulley so as to be rotatably and axially displaceable. The rear side of the rotary ring is formed with a V-shaped curved track. A support ring adjacent the rotary ring likewise comprises a V-shaped curved track. Rolling elements, such as steel balls, are disposed between the tracks. The support ring is rotationally and axially rigid with respect to the driving shaft. Pressure fluid is supplied to the torque sensor in the region of the cylinder-piston assembly by means of a duct. It normally can flow away by way of another duct. The other duct is arranged in such a way that it is overlapped and controlled by one edge of the front side of the rotary ring acting as an annular piston. That control occurs when the rotary ring is rotated relative to the support ring upon a change in torque and is axially displaced by the rolling elements between the V-shaped curved tracks.
In this way, as the torque increases, the discharge duct cross-section is progressively reduced resulting in a control pressure that is proportional to the torque. The control pressure acts upon the main pressure control valve and produces a corresponding increase in the pressure of the system.
Another prior art continuously variable transmission unit for motor vehicles is shown in German Offenlegungsschrift (Laid Open Specification) No. 34 41 789. It has two hydraulically controllable friction disc clutches for engaging a forward gear or a reverse gear by means of a set of gear shifting planet wheels, the planet wheel carrier of which can be coupled by the forward gear clutch to a sun gear connected to the primary shaft of a continuously variable belt-drive transmission. The continuously variable belt-drive transmission has an axially displaceable cone pulley part on the primary shaft which is axially displaceable on the primary shaft. The pulley part that is axially stationary is a cone pulley connected to part of the piston of an hydraulic displacement device. The piston of the displacement device is connected in an axially rigid manner to the primary shaft part of the continuously variable belt-drive transmission. It comprises a stationary cone pulley part and is surrounded by the displacement cylinder secured to the axially displaceable cone pulley part.